Concrete mixing drum manufacturing method

ABSTRACT

A method for making a concrete mixing drum includes forming a first layer of at least one polymeric material having a surface providing at least a portion of a barrel wall of the drum and a spiral mixing blade projecting from the wall on an inner surface of the drum and forming a second layer on the formed first layer.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present continuation application claims priority under 35 U.S.C. §120 from U.S. patent application Ser. No. 10/049,605 filed on Jun. 6,2002 now U.S. Pat. No. 6,902,311 by Anthony J. Khouri and WilliamRodgers, and entitled PLASTICS DRUM FOR CONCRETE MIXING AND METHODS OFMANUFACTURE THEREOF, which is a continuation of co-pending PCTInternational Application No. PCT/AU00/011226, having an Internationalfiling date of Oct. 9, 2000, and having a priority date of Oct. 8, 1999and entitled “VEHICLE MOUNTED PLASTICS DRUM FOR CONCRETE MIXING ANDMETHODS OF MANUFACTURE THEREOF, the full disclosures of which areincorporated by reference.

BACKGROUND

The building industry makes widespread use of concrete mixing trucks fortransportation of ready mixed concrete to sites for concrete pours.These trucks typically comprise a large mixing assembly including amixer drum mounted to the vehicle and which is connected to a mixerdrive for mixing concrete contents during transportation and fordischarge of the contents on site. The drive system comprises a gear boxwhich takes power from the vehicle motor and which applies a mixingtorque to the drum imparting axial rotation to the drum with the torquebeing adjustable depending upon the operating requirements. The abovegeneral arrangement is described in U.S. Pat. No. 4,585,356 whichdiscloses a concrete mixer truck having a mixer drum adapted to berotated by the traction motor of the vehicle through an auxiliarytransmission of the traction motor transmission.

According to the known vehicle mounted mixing assemblies, the mixingdrum is typically of heavy duty steel construction and is disposed atapproximately 10 to 15 degrees from horizontal. The drum is fitted withinternal vanes or mixing blades defining an archimedian spiral so thatas the drum rotates in a first direction the concrete held therein ismixed and as the drum is rotated in the opposite direction, the concreteis discharged from the drum via an elevated discharge orifice under thereverse action of the internal spiral vanes. The drum is disposed suchthat the drive end is lowest and the discharge end is highest relativeto a generally horizontal plane of the vehicle.

While steel drums have been in use for many years, they suffer from anumber of attendant disadvantages, relating to their cost of manufactureand replacement, working life, wear characteristics, weight and volume.

Steel drums are expensive to manufacture due to their labour intensiveconstruction which involves rolling steel sheets into conical portionsand cylinders which once fabricated are then welded to form the finishedtank. The archimedian spirals formed from flat sheets are then weldedinto position inside the drum. As concrete is a highly abrasivematerial, internal surfaces of steel drums are subject to significantwear abrasion. This occurs particularly on the surfaces which take slumpimpact, sliding friction and shear load leading to eventual wearing outof the drum.

Typically, a steel drum used every day might last three to five years,thereafter requiring replacement at significant cost. The abrasion ofinternal surfaces is increased where there are changes in slope in thedrum walls usually where the segments of the drum are joined.

The mixing blades are welded to the internal surface of the drum causingsharp angled recesses in which concrete can gather and eventually buildup degrading the internal surface and providing a catchment for furtherunwanted build up of concrete. By its nature, a steel surface isrelatively smooth and whilst this may be desirable for the purpose ofpreventing concrete build up on the walls of the drum, the interfacebetween the concrete and steel wall is an area of abrasion rather thanconcrete mixing.

Ideally, mixing of concrete should take place throughout the whole mix,but in the steel drums, optimum mixing does not take place at theboundary layer and in crevices in which concrete may collect. In fact,due to the nature of the frictional interface between the steel surfaceand concrete boundary layer, laminar flow occurs resulting in little orno mixing at the boundary layer. The reason for this is that theaggregate in the concrete slides and abrades (with reduced or no mixing)rather than rotates to facilitate mixing. Thus there are ‘dead’ spots inthe mix where no mixing takes place and where there is an increasedpotential for unwanted collection of concrete. In addition to the aboveproblems associated with the use of steel mixing drums, there are costand weight factors which add to inherent inefficiencies in use of steeldrums. Due to the dead weight of the steel drum, its volume must berestricted so the combination of the dead weight and concrete weightmust be maintained within the maximum allowable loading limits for thevehicle to which the drum is attached. The inventor considered thepossibility of using a lightweight material such as plastics forconstruction of a concrete mixing drum as a substitute for steel whilstrecognizing that there were numerous structural and manufacturingdifficulties to be overcome in making the transition to plastics not theleast of which was the production of a drum which could withstand thehigh static and dynamic loadings to which truck mounted mixing drums aresubject to in normal operation. If the weight of the drum could bereduced without compromising and possibly increasing drum volume theweight reduction could be taken up with additional concrete therebyincreasing the pay load.

SUMMARY OF THE INVENTION

The present invention seeks to provide an alternative vehicle mountedrotating cement or concrete mixing drum fabricated from plasticsmaterials which overcomes the aforesaid disadvantages of the prior artand which not only improves the concrete mixing characteristics butprolongs the life of the drum in comparison to its steel equivalent andallows an increase in concrete carrying capacity of the drumcommensurate with the reduction in drum dead weight thereby resulting inan increase in pay load for potentially each vehicle journey and withoutbreach of the boundary requirements of the vehicle.

In the broadest form of the apparatus aspect the present inventioncomprises; a heavy duty rotary concrete mixing drum capable ofattachment to a vehicle; the drum comprising a first end which engages avehicle powered drive assembly which rotates said drum for mixing ofsaid concrete and a second end from which mixed concrete is discharged;wherein said drum is manufactured from at least one layer of plasticsmaterial; wherein the drum includes a wall having integral internalformations which promote mixing and discharge of said concrete and aninner surface which promotes mixing of the concrete.

In one broad form of the apparatus aspect the present inventioncomprises: a vehicle mounted rotary concrete mixing drum having anopening at one end for receiving and discharge of concrete therefrom andat the other end means for engaging a drive assembly so as to rotate thedrum for mixing or discharging concrete; wherein, the drum ismanufactured from at least one mold using at least one plasticsmaterial; wherein the drum further includes detachable or integrallyattached vanes which outstand from the internal surface of the drumforming an archimedian spiral disposed such that when the drum isrotated in a first direction. The concrete contents are mixed and whenthe drum is rotated in a second direction the contents are dischargesfrom said drum; and wherein the internal surface of the drum is formedor lined with an elastomer which causes mixing of the contents of theconcrete at the concrete boundary layer; and wherein the weight of thedrum is such that when full, the total weight of the drum and contentsis lighter than for a steel drum of an equivalent size when full.

In another broad form of the apparatus aspect, the present inventioncomprises:

-   a vehicle mounted rotary concrete mixing drum having an opening at    one end for receiving and discharge of concrete therefrom and at the    other end means for engaging a drive assembly so as to rotate the    drum for mixing and discharging concrete; wherein, the drum is    manufactured from two or three molds using at least one layer of    plastics material; wherein the drum further includes detachable or    integrally attached vanes which outstand from the internal surface    of the drum forming an archimedian spiral disposed such that when    the drum is rotated in a first direction the concrete contents are    mixed and when the drum is rotated in a second direction the    contents are discharged from said drum; and wherein the internal    surface of the drum comprises a polyurethane layer to enhance mixing    of the contents of the concrete at the concrete boundary layer; and    wherein the weight of the drum is such that when full, the total    weight of the drum and contents is lighter than for a steel drum of    an equivalent size when full.

In another broad form of the apparatus aspect, the present inventioncomprises:

-   a vehicle mounted rotary concrete mixing drum having an opening at    one end for receiving and discharge of concrete therefrom and at the    other end means for engaging a drive assembly so as to rotate the    drum for mixing or discharging concrete; wherein, the drum is    manufactured from two or three molds and comprises a first plastics    material such as woven fiberglass forming an outer surface of the    drum and a second plastics material such as polyurethane or like    elastomer forming an inner surface of the drum; wherein the outer    and inner surfaces together form a wall of the drum and wherein the    drum further includes detachable or integrally attached vanes which    extend inwardly from the wall of the drum forming an archimedian    spiral disposed such that when the drum is rotated in a first    direction, the concrete contents are mixed and when the drum is    rotated in a second direction the contents are discharged from said    drum; and wherein the inner polyurethane surface of the drum    provides wear resistance and enhances mixing of the contents of the    concrete at the concrete boundary layer, and wherein the weight of    the drum is such that when full, the total weight of the plastics    drum and contents is lighter than for a steel drum of an equivalent    or smaller size when full.

In a broad form of the method aspect the present invention comprises amethod of manufacture of a vehicle mounted plastics concrete mixing drumcomprising the steps of:

-   a) preparing a mold having a surface defining an internal profile of    said drum which includes a wall having recesses which provide a mold    part for continuous helical mixing blades included in said drum:-   b) applying a release agent to an outer surface of said mold;-   c) applying over said release agent a plastics layer in liquid form    and allowing said plastics layer to set against the mold so as to    form a first layer of a wall of said drum;-   d) applying a bonding layer to said plastics layer;-   e) applying a fiber reinforced composite layer to said bonding    layer; and-   f) removing the mold from the interior of said drum.

A method of manufacture of a vehicle mounted plastics concrete mixingdrum comprising the steps of:

-   a) taking a male mold defining an internal profile of said drum    including a wall having recesses which form a continuous helical    mixing spiral:-   b) applying a release agent to an outer surface of said mold;-   c) applying over said release agent an elastomer in liquid form and    allowing said elastomer to polymerise against the mold so as to form    a first layer of a wall of said drum;-   d) applying a bonding layer to said elastomer;-   e) applying a filament fibre reinforced composite layer to said    adhesive layer;-   f) winding said filament about said drum to form an outer fibre    reinforced structural matrix.

According to another broad form of the method aspect, the presentinvention comprises:

-   a method of manufacture of a vehicle mounted plastics rotatable    concrete mixing drum comprising the steps of:-   a) taking a male mold part whose external surface defines an    internal profile of a concrete mixing drum;-   b) applying a release agent to an outer surface of said mold part;-   c) applying over said release agent an elastomer in liquid form and    allowing said elastomer to polymerise against the mold so as to form    a first layer of said drum;-   d) applying a bonding agent to said elastomeric layer as a substrate    coupling layer to receive an outer structural layer of filament;-   e) winding said filament about said drum to form an outer structural    matrix.

According to another broad form of the method aspect the presentinvention comprises:

-   a method of manufacture of a vehicle mounted concrete mixing drum    comprising the steps of:-   a) fabricating base mold sections for receiving plastics material    used in the formation of said drum;-   b) assembling said base sections mold sections on a mandrel;-   c) clamping said mold sections together to form a helical groove    about said mold sections;-   d) applying a plastics material to the outside of said mold sections    whereby the plastics material forms an interior layer of said drum;-   e) applying an intermediate adhesive layer to the outside of said    interior layer;-   f) applying to said adhesive layer a fibre reinforced outer    structural layer wound about said mold sections wherein said    interior, intermediate and outer layers form a wall of said mixing    drum.

Preferably said mold parts are clamped together prior to application ofsaid interior layer via aligning dowels and an adhesive.

According to a preferred embodiment, the drum is manufactured from threemolded parts two of which comprise end parts of the drum and a thirdcomprising a central part for location between said end parts. Each moldpart has a formation which imparts to the drum part formed by the moldpart, a part spiral extending inwardly from the wall of the drum partsuch that when the drum parts are engaged together, an internalarchimedian spiral is formed. The molds are configured such that whenthe molds are mated together, the internal archimedian spiral used forboth mixing and discharge of concrete from the drum is complete.Preferably, the outer mold surfaces are prepared with a release agent sothe mold may be readily removed after curing. Preferably the elastomeris polyurethane and has a surface property which reduces abrasion yetenhances mixing. Preferably there are three mold sections in which thejunctions form part of the mixing spirals such that the sections arejoined along the spirals.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described according to a preferred butnon limiting embodiment and with reference to the accompanyingillustrations wherein:

FIG. 1 shows a side elevation of a prior art mixing drum;

FIG. 2 shows a side elevation of a cement mixing drum according to oneembodiment of the invention;

FIG. 3 shows three mold parts to be joined in making a drum;

FIG. 4 shows the mold sections of FIG. 3 assembled onto a mandrel;

FIGS. 5 a-d show the first stages of preparation of the drum.

FIG. 6 shows an enlarged profile section of a typical mixing blade.

FIGS. 7 a-c show an end elevation view of the mold clamping andinflation steps.

FIG. 8 shows the mold and drum stored for demolding.

FIG. 9 shows a drum inside a grit chamber in which a grit jet istraversed over the shell surface to prepare the surface so it ischemically receptive to the bonding of the next stage.

FIG. 10 shows the drum mounted for rotation on a computer controlledwinding machine.

FIGS. 11 a-b show a two step process for application of a gel coat.

FIG. 12 shows the drum adapted with a stiffening ring which distributesloads from trunion rollers incorporated on the vehicle on which the drumis to be mounted.

FIG. 13 shows the orientation of the drum during application of the dripring.

FIG. 14 shows a cross section of an end region of a drum includingbetween spiral section and wall a baffle imparting rigidity to the drum.

FIG. 15 shows a cross section of a typical interface between a concretemix and a steel wall.

FIG. 16 shows an enlarged view of the boundary layer wall/concreteinterface in a plastics mixing drum according to a preferred embodimentof the invention.

FIGS. 17 a-o show the various stages in construction of a drum accordingto an alternative embodiment;

FIGS. 18 a-f show the various steps in the construction of a solid coreblade according to an alternative blade arrangement.

FIG. 18 g is a fragmentary sectional view of a drum according to analternative embodiment.

FIGS. 19 a-p show the various stages of construction of a drum accordingto a preferred embodiment.

FIGS. 20 a-f show steps on the construction of a solid core helicalblade according to an alternative embodiment.

FIG. 21 shows a cross sectional view of a solid core blade interactionwith concrete during rotation of the drum.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 shows an elevation view of a known steel mixing drum 1 which istypically constructed from separate prefabricated sections 2, 3 and 4which are welded together at seams 5, 6 and 7. At seams 5, 6 and 7 thewelded joints which are subject to concentrated wear due to the changein surface direction at the joint. The concentrated wear points in theprior art steel drums reduces the working life of the drumsnecessitating costly repair or replacement. Steel drums are fabricatedfrom rolled flat sheets which form cones and a cylinder which are thenjoined together by welding. Archimedian spirals are then welded to theinner surface of the drum resulting in a high specific gravity vesselwhose self weight reduces the amount of concrete which can be carried bythe vehicle to which it is attached. As previously indicated, the steeldrums suffer from a number of disadvantages including susceptibility toabrasion at the junctions of the cylindrical and conical sections andthe tendency for unwanted concrete build up at the sharp corners andcrevices formed by the mixing blades. In addition, the smooth internalsurface of the steel drum promotes sliding abrasion and inhibits mixingat the boundary layer due to the low coefficient of friction at theconcrete/metal interface.

FIG. 2 shows an external profile of a fibre reinforced compositeconcrete mixing drum 8 according to one embodiment of the invention. Thedrum includes an internal archimedian spiral formed by helical blades orvanes which mix concrete during rotation of the drum in one directionand discharge concrete when the drum is rotated in an oppositedirection. The drum is generally pear shaped and includes an opening 9at one end for entry and discharge of concrete. The arrangement of FIG.2 is arrived at by application of the method aspect of the inventionwhich will be described in detail below. Drum 8 is constructed from afibre reinforced plastic structural shell with an elastomeric interiorhaving a surface property which imparts abrasion resistance to theconcrete but increases the mixing at the boundary layer of the concreteand drum wall by forced rotation of aggregate. A preferred method ofconstruction of the drum will now be described in detail. Whilst theembodiment described employs three mold parts it will be appreciatedthat the drum may be constructed from a lesser or greater number ofparts. It has been found however, that a three part construction ispreferable in view of the shape of the finished drum. According to oneembodiment, drum 8 is constructed from three mold sections 13, 14 and 15(shown in FIG. 3) which form portions 12, 11 and 10, respectively, ofdrum 8. FIG. 3 shows typical profiles of mold sections 13, 14 and 15.Portion 12 of drum 8 is constructed to engage a drive system mounted ona vehicle for rotating the drum. Portion 11 extends between portions 12and 10. Portion 10 includes a discharge opening through which mixedconcrete is discharged. The first step in the construction of the mixingdrum is the preparation of the mold from which the drum will beproduced. The mold is preferably constructed in three parts as thisenables ease of extraction form mold formers and also allows for theformation of mixing drums of different sizes according to requirements.For instance the length of the drums can be increased by changing thesize of intermediate section 14. Mold sections 13, 14 and 15 are eachformed in separate molds from expanded polystyrene beads. Steam heat isapplied through slots in each mold thereby fusing the beads against themold surface. The polystyrene surface finish of the mold may be improvedby the application of a fast drying liquid. The external profile of themold parts when joined provide the mold for the inner surface of thedrum. The mold profile includes helical grooves which are the inverse ofhelical mixing blades which extend from an inner surface of the finisheddrum. The mold and partially finished drum are shown at 16.

FIG. 4 shows an assembled mold mounted on a cantilevered mandrel 17. Themold sections include mating dowels which align corresponding parts ofthe mold profiles and are clamped together with an adhesive. FIGS. 5 a-dshow the first stages of preparation of the drum. At the commencement ofthis stage the mold is mounted on mandrel 17 which is capable of axialrotation. The operation of the mold is computer controlled and employs arobot which rotates the mandrel. The mold surface is prepared with arelease agent which will enable release of the mold from the drum oncompletion of the method steps. A spray head 18 delivers a polyurethaneelastomer to the surface of the mold which forms an inner layer of thedrum.

The spray is applied normal to the surface of the drum which will atthis stage be rotating according to parameters fed into the computer. Asthe mold rotates on the mandrel the spray head moves to follow the moldsurface and in particular the path of the grooves. A computer programcontrols the delivery of the polymer to the mold surface. Theapplication of the polyurethane elastomer to the surface of the moldtakes place in two stages. First, the spray is applied to the grooves ofthe mold which will form the helical blades of the drum. Spray head 18follows the contour of the helix about the mold and deposits a uniformcoating onto the sides of the blades with an additional thickness deepin the groove which will form the helical spiral blade tip. Theadditional material provides abrasion resistance during operation. Inthe second part of the spray operation, spray head 18 is changed tospray the polyurethane elastomer normal to the surface of the moldaccording to the required thickness. Additional polyurethane may besprayed where additional thickness is required in areas of high wear. Ifrequired, multiple coats may be deposited in one or both stages.According to one embodiment as shown by FIG. 18 g, one or more of anyadditional layers, such as intermediate layer 75, may be differentlycolored to provide wear indicators. A white pigment in the surface layermay be provided for cleaning and inspection after use. The polyurethaneis allowed to gel following which a chemical layer is sprayed onto thepolyurethane surface to ensure bonding with the next fibre reinforcedcomposite layer.

Prior to application of the fibre reinforced layer, a rope formed ofmultiple glass fibre strands is delivered from a dispensing creel intothe grooves of the spiral helix. This part of the operation isrepresented by FIG. 5 d. The rope is drawn through a bath of resin andis lead through a guide eye to fall into the blade groove. Tensioning ofthe rope pulls it into the groove. When the rope hardens it becomes ahigh strength reinforcing bar along the full length of the helicalspiral. The polyurethane which is sprayed onto the mold by this stagegenerally conforms to shape of the mold except for bridging that isrequired between the groove walls. In the example of FIG. 6, there isshown an enlarged profile section of a typical mixing blade 24. Eachblade comprises an elastomeric layer 20 which forms the inner surface ofthe drum. A coupling layer 21 is applied over the elastomeric layerfollowing which a structural layer 22 is applied inside the concaverecess 23. This process is completed for each section of spiral at ajoin whereupon, a further coupling layer 25 is applied to the remainderof the outer surface of the drum over which is applied a structurallayer 26 which is preferably a fibre reinforced composite to form astructural shell. Included deep inside recess 23 is a continuousfilament and resin rope 35.

A rigid shell is required to bridge across the helical groove and thisis provided by sprayed composite resin and chopped glass strandscompleting the structural layer 26. The sprayed resin is hand rolledfollowed by clamping then inflation of the mold before the polyurethanehas gelled. FIGS. 7 a-c show an end elevation view of the mold clampingand inflation steps. FIG. 7 a shows the clamping assembly 30 in the openconfiguration. Mold and partially completed plastics drum is representedby broken line 31. Before the composite of resin and chopped glassstrands has gelled the mold is located on clamp assembly 30 whereuponarms are closed over the composite. As shown in FIG. 5 c. Afterclamping, the mold is inflated to ensure complete contact with the fibrereinforced composite layer. The mold and drum 31 are stored for fourhours until the resin is sufficiently cured for the next stage. FIG. 8shows the mold and drum 31 stored for demolding. FIG. 9 shows drum 31inside grit chamber 32 in which a grit jet is traversed over the shellsurface to prepare a surface which is chemically receptive to thebonding of the next stage. The next step involves filament winding of afibre reinforced structural layer. A winding arrangement as shown inFIG. 10 is arranged to wind resin wetted fibre rovings around a rotatingformer. The tensile strength of the windings which may be in the orderof 600 Mpa. FIG. 10 shows drum 31 mounted for rotation on a computercontrolled winding machine to enable winding of glass ravings 34. Toobtain the optimum physical properties of the filament wound structurethe fibres are aligned to the loads imposed in use of the finished drum.Typical loadings on the drum are axial bending under weight of wetconcrete, an applied dynamic load at the drive end of the drum, drivingtorque and support loads at the discharge trunion rolls. The windingpattern of the filaments aligns the fibres at 10 degrees at mid span towithstand bending stresses, increasing in angle and in wall thicknesstowards the discharge end to accommodate applied roller loads.

According to one embodiment the winding machine has three motor driveswhich rotate mandrel 17, move carriage parallel to the mandrel axis anda third motion at right angle to this. The rovings which line the drumare drawn through the resin bath and applied to the surface of the drumas a wide ribbon comprising thousand of tensioned fibres. The compositeis applied by winding filament about the drum over the coupling layer 25to form a fibreglass matrix with high strength properties sufficient towithstand normal operating loads applied during mixing and transportingconcrete. The windings overlap until the required thickness is reached.The surface of the drum is covered with wet resin and smallirregularities which need to be addressed to provide the externalfinish. As a result of this construction, the spiral mixing bladesinside the drum are hollow with high bending and shear resistance duringmixing operations. The inner elastomeric surface is highly resistant toabrasion by concrete yet it is softer and lighter than the steelequivalent. The higher resistance to abrasion is facilitated by thenatural elastic deformation of the elastomer which absorbs the kineticenergy of the concrete particles without gouging of the surfacematerial. In addition, due to the property of the inner surface whichwill preferably be polyurethane, the concrete will be mixed rather thanslide at the boundary layer ensuring efficient mixing of the concretethroughout the mix and reduction of abrasion due to the smooth curvesthroughout the interior of the drum. In a further step, the structurallayer is finished with a smooth pigmented resin which is appliedutilising a clamp similar to that used for completion of the resinlayer.

FIGS. 11 a and b show a two step process for application of a gel coat.Shell 40 is larger than shell 30 to accommodate the additional layer ofthe windings. As shown in FIG. 12 drum 31 is adapted with a stiffeningring 43 which distributes loads from trunion rollers incorporated on thevehicle on which the drum is to be mounted. This stage allowsapplication of a corporate livery or alternative indicia into thestructure of the finished drum. To achieve this shell parts 41 and 42are printed with a selected livery and sprayed with a background gelcoat. After gelation a light layer of reinforced composite is appliedand allowed to set. The shells re prepared in advance of the operationof application of the fibreglass windings while the resin is stillliquid whereupon the shells are clamped around the windings therebyextruding out any excess resin. The shell mold assembly is mountedvertically and a two part compound is injected into the track ring moldspace.

FIGS. 13 a-c show the orientation of the drum 31 during this step. Oncethe resin has gelled, the shell molds are removed and the discharge endoverwind is trimmed and a polyurethane drip flange is bonded at thedischarge end. The final step involves removal of the mold remaininginside the drum followed by closure of the mandrel hole and cosmeticfinishing. The mandrel is removed and the hole fitted with a pipeconnection. The drum is stood vertically and acetone which dissolves thepolystyrene is pumped into and out of the interior which is then cleanedand washed. The drum is then finished by removal of any resin flash.

FIG. 14 shows a cross section of an end region of a drum 50 includingbetween spiral section 51 and wall 52 a baffle 53 imparting rigidity tothe drum. The battle plate is preferably glued into position. FIG. 15shows a cross section of a typical interface between a concrete mix 54and a steel wall 55. Due to the inherent smoothness of the steel surface56 the concrete tends to slide and abrade rather than mix FIG. 16 showsa cross section of a typical interface between a concrete mix 57 and anelastomeric boundary layer 58. As shown by arrows 59 the aggregate inthe mix rotates due to the friction between concrete 57 and surface 58.The rotation avoids excessive abrasion of the surface 58 and enhancesconcrete mixing. Furthermore, as surface 58 is able to deflect, energyis dissipated by the inherent elasticity of the surface contributing tothe reduction in wear. According to the preferred embodiment, the spiralblades inside the drum range varying between 0.5 and 2 meter pitch. Atthe drive end of the drum the spirals are approximately 2 meter pitch.The blades are reinforced by chopped strand, woven cloth or filamentwinding. The molds may allow for a variety of helix pitches of theblades. Preferably, the radius of the root of the blade is greater than10 mm to avoid unwanted accumulation of set concrete. Furthermore, theblades are strengthened by their molding integrally with the wall of thedrum and have a stiffness factor which will sustain all applied normaloperating loads. In an alternative embodiment, the internal blades maybe detachably fixed to the wall of the drum.

An alternative method for construction of a fibre reinforced drum isshown in FIGS. 17 a-o. FIG. 17 a shows a profiles of half mold part 60which is coupled with a corresponding half to form completed mold 61.The first step in the construction of the mixing drum is the preparationof the mold from which the drum will be produced. The size of the drummay be changed by changing the dimensions of the mold. Mold sections areeach formed from separate molds from expanded polystyrene beads. Steamheat is applied through slots in each mold thereby fusing the beadsagainst the mold surface. The external profile of the mold parts whenjoined provide the mold for the inner surface of the drum. The moldprofile includes helical grooves which are the inverse of helical mixingblades which extend from an inner surface of the finished drum.

FIG. 17 b shows an assembled mold mounted on a cantilevered mandrel 62.The mold sections include mating dowels which align corresponding partsof the mold profiles and are clamped together with an adhesive. FIG. 17c shows mold 61 at a stage during which the polystyrene surface finishof the mold may be improved by the application of a fast drying liquid.FIGS. 17 d-g show the first stages of preparation of the plastics drumaccording to the embodiment to be described. The mold is mounted onmandrel 62 which is capable of axial rotation. The operation of the moldis computer controlled and employs a robot which rotates the mandrel.The mold surface is prepared with a release agent which will enablerelease of the mold from the drum on completion of the drum. A sprayhead 63 delivers a polyurethane elastomer to the surface of the moldwhich forms an inner layer of the drum.

The spray is applied normal to the surface of the drum which will atthis stage be rotating according to parameters fed into the computer. Asthe mold rotates on the mandrel the spray head moves to follow the moldsurface and in particular the path of the grooves. The application ofthe polyurethane elastomer to the surface of the mold takes place in twostages. First, the spray is applied to the grooves of the mold whichwill form the helical blades of the drum. FIGS. 18 a-f show the variousstages of construction of a solid core blade arrangement according to analternative embodiment. Spray head 63 follows the contour of the helicalgroove 64 about the mold and deposits a uniform coating of polyurethane65 against wall 66 terminating in the region of groove bottom 67. Thelayer applied includes a return portion 68 which provides a bed 69 intowhich is laid continuous glass fibre reinforced elastomer 70. Returnportion 68 will form the helical blade tip and this will be strengthenedby the glass fibre elastomer 70. FIG. 18 c shows an additional layer ofpolyurethane 71 sprayed over glass fibre elastomer 70 thereby completingthe blade profile. The additional material strengthens the blade. In asecond part of the spray operation, spray head 63 is changed to spraythe polyurethane elastomer normal to the surface of the mold accordingto the required thickness. Additional polyurethane may be sprayed whereadditional thickness is required in areas of high wear. If required,multiple coats may be deposited in one or both stages. To ensure thatthe solid core blade profile is retained during the second sprayingoperation, the cavity formed by helical groove 64 is covered bypolyurethane mold insert 72 as shown in FIG. 18 d. A layer ofpolyurethane 73 is then sprayed over insert mold 72 and also over theoutside of drum 61. This is followed by the application of a choppedglass layer 74. FIG. 17 e represents the stage of blade reinforcementand preparation and FIG. 17 f represents the stage of application of thepolyurethane coating over the outside of the drum following completionof the blade profile. According to one embodiment as shown by FIG. 18 g,one or more of any additional layers, such as layer 75, may bedifferentially colored with to provide wear indicators. A white pigmentin the surface layer may be provided for cleaning and inspection afteruse. The polyurethane is allowed to gel following which a chemical layeris sprayed onto the polyurethane surface as represented by FIG. 17 g toensure bonding with the next fibre reinforced composite layer. Acoupling layer is applied to the remainder of the outer surface of thedrum over which is applied a structural layer which is preferably afibre reinforced composite to form a structural shell. A rigid shell isrequired and this is provided by sprayed composite resin and choppedglass strands completing the structural layer. The sprayed resin is handrolled followed by clamping as shown in FIGS. 17 i, j and k.

FIGS. 17 j and k show the clamping assembly 80 in the open and closedconfigurations respectively. Mold and partially completed plastics drum81 is shown in FIG. 17 j. Before the composite of resin and choppedglass strands has gelled the mold is located on clamp assembly 80whereupon arms 82 and 83 are closed over the composite layer. Afterclamping, the mold may be inflated to ensure complete contact with thefibre reinforced composite layer. The mold 61 and drum 81 are stored forfour hours until the resin is sufficiently cured for the next stage.Casting of a track ring and application of a drip flange area aspreviously described. The inner mold is removed as previously describedand this includes removal of mold insert 72. FIG. 18 f shows a typicalsolid core blade profile 84 as described above. The blade satisfiesstrength requirements and is reinforced by curves in the blade profileas the blade traverses the helix about the finished drum interior.Preferred material of construction for the solid core blade will besprayed SP85 polyurethane elastomer (85 shore A). Preferred reinforcingof the solid blade is high tensile glass fibre CC60 elastomer.Preferably, the tensile reinforcement is continuous along the length ofthe blades.

FIGS. 19 a-p show an alternative method of construction of a plasticsdrum including an injection molding step. A number of the stepsaccording to this embodiment are substantially the same as for thecorresponding steps described with reference to FIGS. 17 a-o.

The methods, however differ firstly in relation to the method ofconstruction of the helical blade. FIGS. 19 a-e show a mold 90 mountedon mandrel 91 in the usual manner. Blade reinforcement operationrepresented by FIG. 19 e is shown in more detail in FIGS. 20 a-f.

A spray head (not shown) follows the contour of the helical groove 99about the mold 90 and deposits a uniform bed polyurethane 101 againstcontoured base 102 at the bottom of groove 100. As illustrated in FIG.20 a, bed 101 is trowelled prior to setting with a profiled trowel head103 and this forms a molded recess 104 into which is laid continuousglass fibre reinforced elastomer 105 as shown in FIG. 20 b. Bed 101 willform the helical blade tip and this will be strengthened by the glassfibre elastomer 105 along the length of the helical blade. Thereinforcing elastomer 105 is prior to installation placed in a resinmatrix under tension.

FIG. 20 c shows inserted in groove 100 a polyurethane insert 106 whichleaves a space between the insert and wall 107. The resulting spacedefines the final profile shape of a solid core blade. As shown in FIG.20 d, spacer blocks 111 are applied to the surface 112 of mold 90 overwhich is placed an external mold 108 as shown in FIG. 20 e. The spacerblocks are preferably made of polyurethane which is the same material tobe injected onto the cavity formed by the insert mold 106 and externalmold shell 108. This arrangement corresponds to the steps illustrated byFIGS. 19 f-h. The mold 90 is preferably disposed vertically forinjection molding of the inner layer of the drum.

FIG. 19 g shows mold shell 108 in an open configuration and FIG. 19 hshows mold shell 108 closed for injection molding of polyurethaneelastomer 109. Injection of cold setting polyurethane resin into themold cavity bonds to the extruded elastomer and the matrix of thetensile member and forms the rest of the blade and the elastomericinterior of the concrete mixer. FIG. 19 i shows partially completed drum120 inside grit chamber 121 in which a grit jet is traversed over theshell surface to prepare a surface which is chemically receptive to thebonding of the next stage.

The next step involves filament winding of a fibre reinforced structurallayer. A winding arrangement as shown in FIG. 19 j is arranged to windresin wetted fibre rovings 122 around a rotating former. While the resinis still wet, the gel coated external mold 123 is closed over thestructural shell to form the external surface of the mixer. This moldincludes a track ring for injection of material therein 124 to form atrack ring 125.

Drip ring 126 may then be fitted. The mold 123 is removed to expose thedrum and the internal mold 90 is then dissolved or broken up forremoval. The tensile strength of the windings may be in the order of 600MPa. FIG. 19 j shows drum 120 mounted for rotation on a computercontrolled winding machine to enable winding of glass rovings 122. Toobtain the optimum physical properties of the filament wound structurethe fibres are aligned to the loads imposed in use of the finished drum.Typical loadings on the drum are; axial bending under weight of wetconcrete, an applied dynamic load at the drive end of the drum, drivingtorque and support loads at the discharge trunion rolls. The windingpattern of the filaments aligns the fibres at 10 degrees at mid span towithstand bending stresses increasing in angle and in wall thicknesstowards the discharge end to accommodate applied roller loads.

The rovings which line the drum are drawn through the resin bath andapplied to the surface of the drum as a wide ribbon comprising thousandof tensioned fibres. The composite is applied by winding filament aboutthe drum over the bonding layer to form a fibreglass matrix with highstrength properties sufficient to withstand normal operating loadsapplied during mixing and transporting concrete. The windings overlapuntil the required thickness is reached. The surface of the drum iscovered with wet resin and small irregularities which need to beaddressed to provide the external finish. As a result of thisconstruction, the spiral mixing blades inside the drum are solid withhigh bending and shear resistance during mixing operations. The innerelastomeric surface is highly resistant to abrasion by concrete yet itis softer and lighter than the steel equivalent. The higher resistanceto abrasion is facilitated by the natural elastic deformation of theelastomer which absorbs the kinetic energy of the concrete particleswithout gouging of the surface material. In addition, due to theproperty of the inner surface which will preferably be polyurethane, theconcrete will be mixed rather than slide at the boundary layer ensuringefficient mixing of the concrete throughout the mix and reduction ofabrasion due to the smooth curves throughout the interior of the drum.In a further step, the structural layer is finished with a smoothpigmented resin which is applied utilising a clamp similar to that usedfor completion of the resin layer. FIGS. 19 k and l show a two stepprocess for application of a gel coat. Shell 123 is larger than shell108 to accommodate the additional layer of the windings. As shown inFIGS. 19 m and n drum 120 is adapted with a track ring 124 whichdistributes loads from bunion rollers incorporated on the vehicle onwhich the drum is to be mounted. The stages illustrated in FIGS. 19 kand l allows application of a corporate livery or alternative indiciainto the structure of the finished drum as previously described. Toachieve this, shell parts 123 a and 123 b are printed with a selectedlivery and sprayed with a background gel coat. After gelation a lightlayer of reinforced composite is applied and allowed to set. The shellsare prepared in advance of the operation of application of thefibreglass windings while the resin is still liquid whereupon the shellsare clamped around the windings thereby extruding out any excess resin.The shell mold assembly is mounted vertically and a two part compound isinjected into a track ring mold space. FIG. 19 m shows the orientationof the drum 120 during this step. Once the resin has gelled, the shellmolds are removed and the discharge end overwind is trimmed and apolyurethane drip ring 126 is bonded at the discharge end.

The final step involves removal of the mold remaining inside the drumfollowed by closure of the mandrel hole and cosmetic finishing. Themandrel is removed and the hole fitted with a pipe connection. The drumis stood vertically as illustrated in FIG. 19 o and acetone whichdissolves the polystyrene is pumped out of the interior which is thencleaned and washed. The drum is then finished by removal of any resinflash. FIG. 20 f shows a cross sectional view of a completed blade 110with mold 90 and mold shell 108 removed. The free end of the blade isenlarged relative to the blade thickness to contain the reinforcingtensile member within the elastomer and to protect the tensile fibresfrom abrasion as concrete is mixed.

As an alternative to the hollow blades previously described withreference to FIGS. 5 and 6, solid core blades may be used. As the solidcore blades which are in the form of a two start helix of variablepitch, application of tension to the high strength reinforcing memberwill tend to move the member towards the axis of the mixer. Thismovement is restrained by the radial tension in in the blade material.Concrete loads applied to the blades during mixing and discharge willinduce tension in the tensile member in the blade interior so that theconcrete loads are carried by tension alone on the components in themixer. Because the blade material is a low modulus elastomeric materialand the blade is restrained by the member along its interior edge andthe attachment to the mixer shell along its outer edge the concreteloads will deflect the blade into a sail shaped surface cupped tocontain the concrete. This effect is accentuated by curving the interioredge of the blade in the direction of motion of the concrete towards theclosed end in the mixing zone and the open end in the concrete dischargezone. Preferably, the tensile member is formed of continuous fibres,such as glass, carbon and aramid reinforcing a reason matrix such aspolyurethane, epoxy, polyester or vinylester. The fibres are placedunder light tension during manufacturing process so they are uniformlyloaded to give maximum strength. Reference in the specification toblades includes reference to a single helical blade along the length ofa drum, a two start blade, a multiple start blade arrangement, vanes,paddles and any suitable member for internal mixing of concrete.

The polyurethane elastomer is formed on the exterior of rigid molds.Because the drum is a flask shaped vessel with the outlet smaller thanthe maximum diameter according to one embodiment this molding methodrequires separate molds which can be withdrawn towards the maximumdiameter. In this case the molding must be joined which increasesmanufacturing costs. In an alternative embodiment of the method ofconstruction of the drum a disposable rigid mold of a plastic foam isformed in an external mold. The composite mixer is them formed aroundthe exterior of this foam mold which is then broken up or dissolved toremove it from the drum as previously described.

At the drive end of the drum there is provided a steel ring which ismolded into the drum structure and proportioned to suit drive equipment.The arrangement is such that it will resist relative rotation betweenthe ring and the fibre reinforced drum under applied torque.

The drum also comprises a track ring, which transmits the vessel loadingto the support rollers and is constructed from fibre reinforced plasticformed integral with the structural shell of the vessel. It isanticipated that the plastics drum will outlast its steel equivalentunder the same working conditions by more than 10 years. The wallstrength will be in the order of 600 Mpa at a thickness of approximately8 mm comprising approximately 2-8 mm polyurethane and 2-8 mm fibreglasswinding. According to one embodiment, the elastomeric layers may be ofcontrasting colors to enable detection of wear spots.

A further advantage in the use of plastics for the mixing drums lies inthe thermal properties of the plastics material. Hot conditions areundesirable for concrete mixing as they accelerate hydration reducingconcrete workability which is an essential property required immediatelyfollowing a concrete pour. In very hot climates, the conventional steelvehicle mounted mixing drums conduct high heat loads which increase heatat the concrete boundary layer due to contact with the super heated drumwall causing unwanted accelerated hydration. This phenomenon isdifficult to avoid with steel drums as the conductivity of steel leadsto high conductive heat transfer from the outer skin of the drum to theinner wall which is normally in contact with the concrete. In some hotclimates, ice is placed in the steel drums in an attempt to arresttemperature increase inside the drum. As concrete hydration is anexothermic reaction, it is sensitive to external temperatures.Accordingly it is desirable that the concrete temperature remainsacceptably low to ensure a satisfactory level of workability and toretard hydration. Steel drums heat up significantly and conduct heatthrough their thickness making the concrete vulnerable to the vagariesof temperature variation. Overheating of the concrete mix is a problemto be avoided and has in accordance with one aspect provided a method ofmanufacture of a plastics drum to take the place of the conventionalsteel drums thereby reducing the unwanted effects of high thermalconductivity typical of the steel drums. The plastics drum allows theconcrete to remain workable inside the drum for longer periods comparedto concrete in steel mixing drums under the same external temperatureconditions and transporting concrete. The structural exterior shell issignificantly strengthened by the process of filament winding whichproduces a structure with many times the strength and stiffness ofrandom fibre composites. At the drive end of the drum the steel ringwhich is molded into the drum structure is proportioned to suit driveequipment. The arrangement is such that it will resist relative rotationbetween the ring and the fibre reinforced drum under applied torque. Thetrack ring, transmits the vessel loading to the support rollers and isconstructed from fibre reinforced plastic formed integral with thestructural shell of the vessel.

It will be recognised by persons skilled in the art that numerousvariations and modifications may be made to the invention as broadlydescribed herein without departing from the overall spirit and scope ofthe invention.

1. A method for making concrete mixing drum, the method comprising:forming a first layer of at least one polymeric material, the firstlayer including at least a portion of a barrel wall of the drum and aspiral mixing blade, the spiral mixing blade projecting from the wall onan inner surface of the drum; and forming a second layer on the formedfirst layer.
 2. The method of claim 1 wherein at least a portion of thespiral mixing blade has a pitch of about 2 meters.
 3. The method ofclaim 2 wherein the spiral mixing blade has a radial mid-portion havinga cross-sectional thickness formed from a single homogenous polymericmaterial.
 4. The method of claim 3 including embedding a reinforcementmember within the spiral mixing blade.
 5. The method of claim 1 whereinthe step of forming the second layer includes continuously extending thesecond layer as a single integral unitary body from a first axial end ofthe drum across an axial midpoint of the drum.
 6. The method of claim 5including continuously extending the second layer as a single integralunitary body to proximate a second axial end of the drum.
 7. The methodof claim 1 wherein the step of forming the first layer includescontinuously extending the first layer as a single integral unitary bodyfrom a first axial end of the drum across an axial midpoint of the drum.8. The method of claim 7 including continuously extending the firstlayer as a single integral unitary body to proximate the second axialend of the drum.
 9. The method of claim 1 wherein the second layerincludes a fiber reinforced plastic.
 10. The method of claim 9 whereinthe second layer is formed by winding the fiber reinforced plastic aboutan axial center line of the drum.
 11. The method of claim 10 wherein thefiber reinforced plastic is wound in a pattern such that fibers of thefiber reinforced plastic are aligned at a first angle at an axialmidpoint of the drum and at a second larger angle towards a dischargeend of the drum.
 12. The method of claim 11 wherein the first angle is10 degrees.
 13. The method of claim 10 wherein the fiber reinforcedplastic wound about the axial center line of the drum has a firstthickness at an axial midpoint of the drum and a second larger thicknesstowards the discharge end of the drum.
 14. The method of claim 10wherein the fiber reinforced plastic being wound about the axial centerline of the drum comprises a resin wetted fiberglass roving.
 15. Themethod of claim 10 wherein windings of the fiber reinforced plastic areoverlapped with one another.
 16. The method of claim 10 wherein thefiber reinforced plastic is continuously wound from a first axial end ofthe drum across an axial midpoint of the drum.
 17. The method of claim16 wherein the fiber reinforced plastic is continuously wound from thefirst axial end of the drum to proximate a second axial end of the drum.18. The method of claim 1 wherein the first layer includes aurethane-based material.
 19. The method of claim 1 including the step offorming a spiral mixing blade on the first layer so that the bladecontinuously extends as a single integral unitary body from a firstaxial end of the drum across an axial midpoint of the drum.
 20. Themethod of claim 19 including continuously extending the spiral mixingblade as a single integral unitary body to proximate a second axial endof the drum.
 21. The method of claim 19 wherein the step of forming aspiral mixing blade on the first layer includes integrally forming atleast a portion of the spiral mixing blade as part of a single integralunitary body with the first layer.
 22. A method for making a concretemixing drum, the method comprising: forming a first layer of at leastone polymeric material, the first layer including at least a portion ofan inner surface of the drum and at least a portion of a spiral mixingblade; and forming a second layer on the formed first layer, wherein atleast one of the first layer and the second layer continuously extendsas a single integral unitary body from a first axial end of the drumacross an axial midpoint of the drum.
 23. The method of claim 22 whereinthe first layer continuously extends as a single integral unitary bodyfrom the first axial end of the drum across an axial midpoint of thedrum.
 24. The method of claim 23 wherein the first layer continuouslyextends as a single integral unitary body to proximate a second axialend of the drum.
 25. The method of claim 24 wherein the second layercontinuously extends as a single integral unitary body from the firstaxial end of the drum to proximate the second axial end of the drum. 26.The method of claim 22 wherein the second layer includes a fiberreinforced plastic.
 27. The method of claim 22 wherein the second layerincludes a fiber reinforced plastic, wherein the method farther includescontinuously winding the fiber reinforced plastic from a first axial endof the drum across an axial midpoint of the drum.
 28. The method ofclaim 27 including continuously winding the fiber reinforced plastic toproximate a second axial end of the drum.
 29. The method of claim 22wherein the spiral mixing blade portion of the first layer continuouslyextends as a single integral unitary body from a first axial end of thedrum across an axial midpoint of the drum.
 30. The method of claim 29including continuously extending the spiral mixing blade as a singleintegral unitary body to proximate a second axial end of the drum.
 31. Amethod of manufacture of a plastics concrete mixing drum comprising thesteps of: a) preparing a mold having a surface defining an internalprofile of said drum which includes a wall having recesses which providea mold part for continuous helical mixing blades included in said drum;b) applying a release agent to an outer surface of said mould; c)applying over said release agent a plastics layer in liquid form andallowing said plastics layer to set against the mould so as to form afirst layer of a wall of said drum; d) applying a bonding layer to saidplastics layer; e) applying a fibre reinforced composite layer to saidbonding layer; and f) removing the mold from the interior of said drum.32. A method according to claim 31 comprising the further step prior tostep a) of forming said mold from expanded polystyrene.
 33. A methodaccording to claim 31 comprising the further step of placing a fibrereinforced member along a concave tip of said helical mixing blade. 34.A method according to claim 31 comprising the further step prior toapplication of said fibre reinforced composite layer of moving said drumformed about said mold to a grit blasting chamber wherein a grit jet istraversed over the drum to prepare an outer surface of said drum with abonding agent to receive said outer layer.
 35. A method according toclaim 31 wherein the drum is manufactured from three mold parts, two ofwhich form end parts of the drum and a third forming a central part forlocation between said end parts.
 36. A method according to claim 35wherein each mold part has a formation which imparts to the drum partformed by the mold part, a part spiral extending inwardly from the wallof the drum part such that when the drum parts are engaged together, aninternal archimedian spiral is formed wherein the molds are configuredsuch that when the drum parts formed from the molds are mated together,an internal archimedian spiral used for both mixing and discharge ofconcrete from the drum is formed.
 37. A method according to claim 36wherein the outer mold surfaces are prepared with a release agent so thesurface of the plastics layer may be separated after curing.
 38. Amethod according to claim 31 wherein said plastics layer is an elastomeris selected from polyurethane and like materials and has a surfaceproperty which reduces surface abrasion on the inner surface of saiddrum but promotes mixing of said concrete.
 39. A method of manufactureof a vehicle mounted concrete mixing drum comprising the steps of: a)fabricating base mold sections for receiving plastics material used inthe formation of said drum; b) assembling said base mold sections; c)clamping said mold sections together to form a helical groove about saidmold sections; d) applying a plastics material to the outside of saidmold sections whereby the plastics material forms an interior layer ofsaid drum; e) applying an intermediate bonding layer to the outside ofsaid interior layer; f) applying to said bonding layer a reinforcedouter structural layer wound about said mold sections wherein saidinterior, intermediate and outer layers form a wall of said mixing drum.40. A method according to claim 39 wherein said mold parts are clampedtogether prior to application of said interior layer via aligning dowelsand an adhesive.
 41. A method according to claim 39 wherein said moldsections are fabricated from polystyrene beads.
 42. A method accordingto claim 39 wherein said interior layer is formed by a polyurethaneelastomer sprayed onto said mold sections.
 43. A method according toclaim 39 comprising the further step of immersing a composite member ofglass fibre strands in a resin bath and winding said member into saidgrooves to reinforce a tip of said helical groove.
 44. A methodaccording to claim 43 comprising the further step of bridging saidhelical groove with a fibre reinforced composite of resin and choppedglass strands and applying said composite to the surface of said bondinglayer, hand rolling said strands and clamping about the exterior of saidmolds prior to setting of said resin and chopped glass strand.
 45. Amethod according to claim 44 comprising the further step of allowing theresin of said composite member to cure for a predetermined period.
 46. Amethod according to claim 44 wherein filaments of said fibre reinforcedcomposite are wound about and along said drum at angles of 10 degrees orgreater and to a predetermined thickness.
 47. A method according toclaim 44 comprising the further steps of: i) preparing a printed layeron a second mold shell and covering said layer with a gel coat; ii)applying to said gel coat a light layer of fibre reinforced composite;whereupon while said resin of said composite is still liquid said shellis clamped about said drum until excess resin is squeezed from saidshell.
 48. A method according to claim 47 including the further step ofinjecting a compound into said mold shell to form a stiffening ring atthe location of loads applied by drum support rollers.
 49. A methodaccording to claim 44 comprising the further step of allowing the resinof said fibre reinforced outer structural layer composite to cure for apredetermined period.
 50. A method according to claim 39 wherein thereare three mold sections which when joined together include a helicalscrew.
 51. A method according to claim 50 comprising the further step ofspraying a wall of said groove with polyurethane to a predeterminedthickness along said wall forming a blade profile terminating in a freeend, wherein said blade profile includes at its free end a returnforming a groove in which is placed a continuous glass fibre reinforcedelastomer which reinforces said blade along the length of said blade.52. A method according to claim 51 comprising the additional step ofspraying additional polyurethane over said reinforcing glass fibreelastomer to embed said elastomer in said polyurethane.
 53. A methodaccording to claim 39 comprising the further step, prior to applicationof said fibre reinforced composite layer, of moving said drum formedabout said mold to a grit blasting chamber wherein a grit jet istraversed over the drum to prepare an outer surface thereof with abonding agent to receive said outer layer.
 54. A method according toclaim 39 wherein the drum is manufactured from three mold parts, two ofwhich are configured to mold end portions of the drum and a thirdconfigured to mold a central portion between said end portions.
 55. Amethod according to claim 54 wherein each mold part includes a recessedformation, said recessed formation imparting to the drum an internalarchimedian spiral for both mixing and discharge of concrete from thedrum.
 56. A method according to claim 55 comprising the further step ofplacing an insert mold over said recesses and spraying polyurethaneelastomer over said insert mold and over the outer surface of said moldto form a continuous layer of polyurethane.
 57. A method according toclaim 56 comprising a further step prior to spraying said polyurethaneof application of a release agent so said polyurethane layer may beseparated from said mold.
 58. A method according to claim 39 whereinsaid mold parts are mounted on a mandrel and rotated during applicationof said interior layer of said drum.
 59. A method according to claim 39wherein said interior layer of said drum is prepared in two steps, thefirst of which comprises the application of said plastics material tothe wall of said groove formed in said mold parts and the second ofwhich comprises application of said plastics material to the remainderof said mold surface.
 60. A method according to claim 59 wherein saidpolyurethane is applied in said first step using a spray head whichtraverses a path which generally aligns with said helical recesses assaid mandrel rotates thereby applying a uniform distribution ofpolyurethane along said wall of said recesses, and wherein said secondstep is performed using a second spray head which traverses a pathnormal to said mold surface.
 61. A method according to claim 59comprising the further step of inflating the mold and said interiorpolyurethane layer to ensure complete contact with and adhesion to saidfibre reinforced composite.
 62. A method according to claim 59comprising the further step of grit blasting said fibre reinforcedcomposite prior to application of an outer structural layer.
 63. Amethod according to claim 59 wherein said first and/or second stepsapplied in the application of said interior layer are repeated at leastone more time.
 64. A method according to claim 39 comprising the furthersteps of forming a bed of polyurethane in said recesses which form saidhelical mixing blades, trowelling said bed to form a groove therein andlaying in said bed a continuous glass fibre reinforced elastomer toreinforce said mixing blades along the length of said mixing blades. 65.A method according to claim 64 comprising the additional step ofinserting a polyurethane insert mold into said recesses such that aspace is formed therein having a blade profile.
 66. A method accordingto claim 65 comprising the additional step of setting an external moldat a predetermined spacing from said drum mold and injecting into saidexternal mold polyurethane which fills said space bounded by said insertmold and said spacing between said drum mold and said external mold. 67.A method according to claim 66 wherein said space bounded by said insertmild when filled forms a solid core blade profile.
 68. A methodaccording to claim 39 wherein said plastics layer is a polyurethaneelastomer and said intermediate adhesive bonding layer is sprayed ontosaid polyurethane elastomer surface after said polyurethane elastomersurface has gelled.
 69. A method according to claim 39 including thestep of colouring said drum layers as wear indicators.
 70. A methodaccording to claim 39 wherein said mold sections are polystyrene, andcomprising the further step of filling said polystyrene mold sectionswith a chemical which dissolves said polystyrene enabling release ofsaid mold from said drum.